1. Field of the Invention
The present invention relates to an electronic circuit storage case provided with a ventilation portion that lessens an internal pressure change in a resin case occurring with an ambient temperature change, and to a manufacturing method thereof.
2. Background Art
When an electronic circuit storage case used for an electronic control unit installed in an engine room of an internal combustion engine is subject to high temperature in an actual use environment, an internal pressure of the case rises with a rising ambient temperature. In this instance, because a water-proof case portion has a sealed structure, assuming that there is no pressure escape, stress concentrates on a thin portion, a joint or the like of a case member and may possibly cause damage. Hence, in order to lessen this pressure, it is typical to provide a ventilating hole connecting the interior and the exterior of the case portion.
It should be noted, however, that a structure to release a pressure directly to the exterior from the case portion is contrary to the waterproof structure. It is therefore necessary to connect the ventilating hole to the interior using, for example, a connector portion.
In addition, because various electronic control units used in the engine room of the internal combustion engine are disposed in a narrow space, a layout thereof is limited and so is an insertion direction of a connector. It is therefore necessary to set an orientation of the connector portion by bending the connector portion with respect to an electronic control unit main body.
For an electronic circuit storage case provided with a ventilation portion, there is a related art technique described, for example, in Patent Document 1. A method adopted in this reference document is as follows. Firstly, a communication hole having opening holes respectively opening in three directions is molded using a die used to mold a case. After resin is allowed to cool and solidify, the molded resin is removed from the die. Subsequently, in a secondary processing work different from the molding, one of the opening holes respectively opening in three directions is closed by melting resin in the vicinity of this opening hole using a heated jig and letting the melted resin run into the opening hole. Further, an adhesive is applied on the top and the periphery of the closed opening hole and allowed to cure. Provided by this method is a communication hole allowing a communication from the exterior of the connector to the interior of the case and of a structure having two turns bent substantially at right angles in the interior.
There is another related art technique described, for example, in Patent Document 2. A method adopted in this reference document is as follows. That is, a ventilating hole is provided in a resin solid portion of a case portion. The ventilating hole has turns at two points and allows the case portion and a connector to communicate. The ventilating hole also has two spaced-apart opening holes opening in two directions to the exterior. An opening portion opening to an exterior of the case portion is provided by extending one end of a ventilating channel allowing a communication between the two turns. This opening is then closed by pre-heating resin in the vicinity of the opening portion and pushing the resin into the opening portion.    Patent Document 1: JP-A-2004-55829 (pp. 5-6, FIGS. 6 and 7)    Patent Document 2: JP-A-2008-182089 (pp. 4-6, FIGS. 4 through 14)
However, the methods of closing one of the opening holes respectively opening in three directions by melting the resin in the vicinity of the opening hole as in the related art techniques described in Patent Document 1 and Patent Document 2 described above have problems as follows. That is, the ventilating hole may be filled with excessively melted resin running therein from the opening hole and the resin may further pass through the opening hole and run into the ventilating hole.
Because the opening holes and the ventilating hole have to be formed in a limited space within the electronic control unit, each is normally formed to have a small passage cross section in the order of φ1 to 2 mm. Hence, even a slight amount of melted resin running into the ventilating hole is enough to close the ventilating hole. Further, in a case where an adhesive is applied on the top and the periphery of the closed opening hole, when the opening hole is not completely closed with the melted resin in the vicinity of the opening hole and a clearance is left in the preceding process, the adhesive runs into the opening hole through the clearance when the adhesive is applied thereon after the preceding process is completed. The adhesive may pass by the resin inside the opening hole and run into the ventilating hole, thereby closing the ventilating hole. In either case, the interior of the case portion is hermetically closed and it becomes impossible to lessen an internal pressure change occurring with a temperature change. Hence, there is a problem that the related art techniques may fail to achieve the intended purpose of the ventilating hole.